WASHINGTON, Oct 29 (Reuters) - With one week left in thetight battle for the White House, it's all about the vital swingstate of Ohio. Unless it's about Colorado - or Iowa, or tiny NewHampshire.

Democratic President Barack Obama and Republican challengerMitt Romney both have clear paths to the 270 electoral votesneeded for victory - and they don't all go through Ohio, thestate that both sides have long viewed as key to capturing theWhite House.

Obama still has a slight electoral map advantage fueled byhis slim lead in Ohio, but Romney has steadily closed the gap ormoved slightly ahead in some other battleground states. Eightstates remain relative toss-ups.

Both candidates can construct multiple winning scenarios,with or without Ohio. And it's now possible that the tippingpoint could emerge from another battleground, such as Colorado,where Obama and Romney are deadlocked in the polls.

"At this point, there are probably more electoral mapscenarios than there are undecided voters," said Lee Miringoff,a pollster at Marist College, which is conducting surveys in keyswing states.

"In a 50-50 race ... everything and everywhere is going tomatter," he said.

National polls show the race is a virtual dead heat, butObama still has a lead of at least 4 percentage points in statesthat account for 237 electoral votes, according to averagescompiled by RealClearPolitics. Romney has a lead of at leastthat size in states that represent 201 electoral votes.

That gives Obama slightly more leeway in the fight for theremaining 95 electoral votes available in the eight toss-upstates, all won by Obama in the 2008 election - Colorado (9electoral votes), Florida (29), Iowa (6), Nevada (6), NewHampshire (4), Ohio (18), Virginia (13) and Wisconsin (10).

Obama is clinging to slight poll leads - which typically areless than the polls' margins of error - in five of those stateswith a combined 44 electoral votes: Ohio, Iowa, Nevada, NewHampshire and Wisconsin. That would be enough to put him overthe top.

Even if he loses Ohio, Obama could still get to 270electoral votes - and clinch the election - by winning Coloradoinstead. Obama won Colorado by 9 percentage points in 2008,aided by support from young and suburban voters and the growingHispanic vote, but he is virtually tied with Romney there now.

Romney's path is tougher without Ohio, but still possible.

The former Massachusetts governor has a slight lead overObama in Florida and has pulled even with the president inVirginia. If Romney sweeps those two states and adds Colorado,he would still need to win Iowa, New Hampshire and Wisconsin tocapture the White House.

NEVADA SLIPPING AWAY FROM ROMNEY?

Of the eight toss-up states, Nevada appears the leastcompetitive, with analysts and some strategists in both partiessaying it is moving toward Obama.

An NBC/Wall Street Journal/Marist poll on Thursday gaveObama a 3-point edge in Nevada, and the last six public pollshave shown Obama ahead.

Romney appears to have an advantage in Florida, where six ofthe last seven public polls have shown him with a small lead.RealClearPolitics puts Romney's average lead at 1.8 percentagepoints, within most polls' margin of error but symbolic of atrend toward the Republican, analysts say.

"Once an incumbent loses a grip on the race, it's very hardto get it back," said Florida-based pollster Brad Coker ofMason-Dixon. "Florida is gone for Obama, from what I'm seeing onthe ground here. The map seems to be expanding for Romney andshrinking for Obama."

The multiple electoral scenarios have sparked speculationabout alternative outcomes such as a 269-269 tie in electoralvotes, which would leave the presidency to a vote by theRepublican-led House of Representatives.

Another possibility: one candidate wins the nationwidepopular vote, while the other wins the electoral vote - andwalks away with the presidency.

The most heavily contested prize remains Ohio, and bothcampaigns are concentrating their time and resources there.Obama has an average lead in polls there of 1.9 percentagepoints, according to RealClearPolitics. Six of the last ninepublic polls showed Obama with a slight edge.

The other three showed a tie, including a poll released onSunday by the Cincinnati Enquirer newspaper and the Ohio NewsOrganization.

"The electoral map tilts slightly to Obama, but only becauseOhio is so important and that's one state where he has kept avery small lead," said Thomas Riehle, a pollster at the marketresearch firm YouGov, which also is surveying swing states.

"The polling is so much closer than it was in 2000 or anyother close election year, so everything is hard to predict," hesaid.